# std::forward

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< cpp‎ | utility

Defined in header `<utility>` |  |
| (1) |
template< class T > T&& forward( typename std::remove_reference<T>::type& t ) noexcept; |  | (since C++11) (until C++14)
template< class T > constexpr T&& forward( std::remove_reference_t<T>& t ) noexcept; |  | (since C++14)
| (2) |
template< class T > T&& forward( typename std::remove_reference<T>::type&& t ) noexcept; |  | (since C++11) (until C++14)
template< class T > constexpr T&& forward( std::remove_reference_t<T>&& t ) noexcept; |  | (since C++14)

1) Forwards lvalues as either lvalues or as rvalues, depending on T.

When t is a forwarding reference (a function argument that is declared as an rvalue reference to a cv-unqualified function template parameter), this overload forwards the argument to another function with the value category it had when passed to the calling function.

For example, if used in a wrapper such as the following, the template behaves as described below:

    template<class T>
    void wrapper(T&& arg)
    {
        // arg is always lvalue
        foo(std::forward<T>(arg)); // Forward as lvalue or as rvalue, depending on T
    }

- If a call to `wrapper()` passes an rvalue `std::string`, then `T` is deduced to `std::string` (not `std::string&`, `const std::string&`, or `std::string&&`), and `std::forward` ensures that an rvalue reference is passed to `foo`.
- If a call to `wrapper()` passes a const lvalue `std::string`, then `T` is deduced to `const std::string&`, and `std::forward` ensures that a const lvalue reference is passed to `foo`.
- If a call to `wrapper()` passes a non-const lvalue `std::string`, then `T` is deduced to `std::string&`, and `std::forward` ensures that a non-const lvalue reference is passed to `foo`.

2) Forwards rvalues as rvalues and prohibits forwarding of rvalues as lvalues.

This overload makes it possible to forward a result of an expression (such as function call), which may be rvalue or lvalue, as the original value category of a forwarding reference argument.

For example, if a wrapper does not just forward its argument, but calls a member function on the argument, and forwards its result:

    // transforming wrapper
    template<class T>
    void wrapper(T&& arg)
    {
        foo(forward<decltype(forward<T>(arg).get())>(forward<T>(arg).get()));
    }

where the type of arg may be

    struct Arg
    {
        int i = 1;
        int  get() && { return i; } // call to this overload is rvalue
        int& get() &  { return i; } // call to this overload is lvalue
    };

Attempting to forward an rvalue as an lvalue, such as by instantiating the form (2) with lvalue reference type T, is a compile-time error.

### Notes

See template argument deduction for the special rules behind forwarding references (`T&&` used as a function parameter) and forwarding references for other detail.

### Parameters

t | - | the object to be forwarded

### Return value

static_cast<T&&>(t)

### Complexity

Constant.

### Example

This example demonstrates perfect forwarding of the parameter(s) to the argument of the constructor of class `T`. Also, perfect forwarding of parameter packs is demonstrated.

Run this code

    #include <iostream>
    #include <memory>
    #include <utility>

    struct A
    {
        A(int&& n) { std::cout << "rvalue overload, n=" << n << '\n'; }
        A(int& n)  { std::cout << "lvalue overload, n=" << n << '\n'; }
    };

    class B
    {
    public:
        template<class T1, class T2, class T3>
        B(T1&& t1, T2&& t2, T3&& t3) :
            a1_{std::forward<T1>(t1)},
            a2_{std::forward<T2>(t2)},
            a3_{std::forward<T3>(t3)}
        {}

    private:
        A a1_, a2_, a3_;
    };

    template<class T, class U>
    std::unique_ptr<T> make_unique1(U&& u)
    {
        return std::unique_ptr<T>(new T(std::forward<U>(u)));
    }

    template<class T, class... U>
    std::unique_ptr<T> make_unique2(U&&... u)
    {
        return std::unique_ptr<T>(new T(std::forward<U>(u)...));
    }

    auto make_B(auto&&... args) // since C++20
    {
        return B(std::forward<decltype(args)>(args)...);
    }

    int main()
    {
        auto p1 = make_unique1<A>(2); // rvalue
        int i = 1;
        auto p2 = make_unique1<A>(i); // lvalue

        std::cout << "B\n";
        auto t = make_unique2<B>(2, i, 3);

        std::cout << "make_B\n";
        [[maybe_unused]] B b = make_B(4, i, 5);
    }

Output:

    rvalue overload, n=2
    lvalue overload, n=1
    B
    rvalue overload, n=2
    lvalue overload, n=1
    rvalue overload, n=3
    make_B
    rvalue overload, n=4
    lvalue overload, n=1
    rvalue overload, n=5

### See also

move (C++11) | converts the argument to an xvalue (function template)
move_if_noexcept (C++11) | converts the argument to an xvalue if the move constructor does not throw (function template)

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